Electronic Structure of Excited States and their Applications in Photoredox Catalysis

Overconsumption of nonrenewable fuels has yielded excessive greenhouse gas emissions; fueling global climate change. Using solar energy to drive thermodynamically arduous reactions has emerged as an approach that stores this energy in the chemical bonds of synthesized molecules termed ‘solar fuels.’ These reactions are driven by light absorbing molecules or materials that can harvest solar energy and drive electron transfer reactions from excited states. This dissertation is aimed at understanding the fundamental electronic structure of molecules in their excited state and the electron transfer reactions that these excited states can perform. CHAPTER 1 describes the theoretical basis and application for the research presented herein. CHAPTER 2 and CHAPTER 3 are aimed at understanding how preassociation of photosensitizers and quenchers impact excited state photophysics and the efficacy of electron transfer. Both chapters provide new insights into the stabilization of ions by Coulombic and hydrogen bonding interactions. Similarly, CHAPTER 4 aims to understand fundamental parameters for electron transfer reactions by experimentally measuring the electronic coupling of a bimolecular reaction for the first time, providing a method to probe the encounter complex. In order for electron transfer products to perform the desirable fuel forming reactions, they must escape the encounter complex. CHAPTER 7 reviews the literature on the factors that dictate cage escape yields and provides design principles for photoredox reactions.CHAPTER 5 and CHAPTER 6 aim to understand how excited state symmetry is either broken or maintained with light absorption. CHAPTER 5 demonstrates the first experimental evidence for a quadrupolar LMCT excited state, previously limited to organic chromophores. CHAPTER 6 presents a hypothesis for how metal-involved charge transfer excited states can delocalize between identical ligands and the parameters that may drive symmetry breaking excitation. By reviewing the literature, in comparison to the localized MLCT of RuII(bpy)32+ and delocalized LMCT of ZrIV(PDP)2, we propose a general hypothesis predicting that vacancies in the metal t2g orbitals facilitate the excited state delocalization that maintains the ground state symmetry with light absorption. These chapters provide new perspectives for how to think about excited state charge redistribution and a possible means to predict excited state symmetry.